Nobody looks at a hinge, and every door depends on one. Antique hinges carry the entire weight of a door through every open and close, thousands of times a year for a century, and they do it silently enough that people forget they're there.
From visible strap to hidden butt
The forms tell their own story. Early doors hung on hand-forged strap and H- and HL-hinges, long decorative shapes spread across the door face, both structural and visible, made when a hinge was a blacksmith's work. As doors and hardware standardized, the butt hinge took over, the compact rectangular hinge mortised into the door's edge, hidden when the door is closed. That's the hinge most people picture without realizing it. The Victorian era brought decoration back to the humble hinge, casting ornamental patterns on the visible barrel and leaves and capping the pin with a finial, so even this workhorse part carried period detail.
The practical matter is matching and weight rating. A hinge has to suit the door's size and weight, and the set has to match, three hinges on a heavy door, all alike. A door rehung on mismatched or underrated hinges sags or binds, so the right hinges are part of hanging a door properly.
Salvager's Note: Match hinges to the door's weight and buy a full matching set (usually three for a full-size door). Mixed or underrated hinges make a heavy door sag and bind, no matter how good the door is.